A paper sheet counting device for calculating the number of passbooks held stacked in a holding portion is conventionally known (see Patent Literature 1, for example). The paper sheet counting device described in Patent Literature 1 includes two reflective optical sensors each having a light emitting element and a light receiving element. The two optical sensors are arranged parallel to a side corresponding to the back of a passbook. Also, the two optical sensors are able to move in the thickness direction of a passbook held in the holding portion. This paper sheet counting device calculates the number of passbooks held in the holding portion, on the basis of the level (peaks of output signals) of output signals (that is, output signals from the light receiving elements) from the optical sensors when the optical sensors pass the position where the passbook is arranged, and the level (bottoms of the output signals) of the output signals from the optical sensors when the optical sensors passes between stacked passbooks.